The present invention relates to the analysis of video sequences for display on interlaced systems such as those according to BT.601 [1]. In such systems, the use of incorrect field ordering (top-field-first (TFF) or bottom-field-first (BFF) can result in noticeable and objectionable effects in the display of both interlaced and progressive-type sequences.
According to the present invention there is provided a method of detecting field order of a video signal, comprising:
receiving successive digitally coded frames, each frame comprising data for a field of a first type and data for a field of a second type;
generating for each field of the first type:                a first difference signal (DIF1) representative of the difference between the field and the second-type field of the previous frame;        a second difference signal (DIF2) representative of the difference between the field and the second-type field of the same frame;        a third difference signal (DIF3) representative of the difference between the field and the second-type field of the following frame; and        in dependence of the values of said difference signals, a decision signal (wOFlag) indicating an estimated temporal relationship of the field to the second-type field of the same frame.        
Other aspects of the invention are set out in the claims.